1. Field of the Invention
The present invention is relative to an optical pickup device used for an optical information recording and reproduction device that reads and writes signals from and onto an optical information recording medium such as an optical disk, and particularly is relative to an aperture control device that is disposed in the optical path, extending from a light source to the optical disk, of the optical pickup device.
2. Description of the Related Art
The consideration to narrow the track pitch on an optical disk is given for the purpose of improving the density of recording onto the optical disk. In order to perform recording or reproduction of information onto or from the optical disk with a narrow track pitch, it is necessary to increase the numerical aperture of an objective lens included in the optical system in proportion to the narrowing of the track pitch. When the numerical aperture of the objective lens is increased, the amount of aberration that arises increases as a result of the tilting of the optical disk. To avoid the increase of aberration, a method is taken by which, for example, the pickup angle is moved to follow the tilt of the optical disk to eliminate the influence of the tilt of the optical disk. In the tilts of surface of the optical disk, a tilt component in the radial direction is due to the deflection of the optical disk. Since the tilt component in the radial direction generally does not vary suddenly, the pickup can be made to follow the optical disk this tilt component by use of a tilt mechanism that uses, for example, a gear and a motor. On the other hand, a tilt component in the tangential direction of the optical disk, in other words, the tilt in the direction of the time axis, varies in proportion to the rotation speed of the optical disk. Therefore, it is difficult to make the pickup follow this tilt component in the tangential direction of the optical disk.
Thus as a rational selection, the numerical aperture in the time axis direction may be set smaller than the numerical aperture in the radial direction so as to change the optical cutoff spatial frequency according to direction. Since the allowable width for the optical disk tilt in the time axis direction will then be enlarged with respect to that of the radial direction, it becomes possible to accommodate the optical disk with tilt using just a tilt mechanism for the radial direction. Especially in cases where it is premised that information from a recorded optical disk will be reproduced using a pickup with a small numerical aperture specialized for reproduction, this method can be considered for applications in which recording is performed using a numerical aperture which is large only in the radial direction in the recording process.
In addition to a method in which an aperture restriction of a circular form is applied to a light beam that passes through an objective lens at the pupil of the objective lens or to the cross section of the light beam that passes through the objective lens, arrangements for changing the magnitude of the aperture through which light passes correspond to the direction of the optical disk include, for example, an arrangement using an elliptical aperture as disclosed in Japanese Laid-open Patent Publication No. Hei 8-335329 (Japanese Patent Application No. Hei 7-141965). Though this method is satisfactory for realizing an aperture that is enlarged in the time axis direction as in the arrangement of this priority application, when this arrangement is to be applied as an aperture that is enlarged in the radial direction, the following problem occurs with regard to the tracking servo for following the tracks onto which recording is to be performed using the pickup.
That is, in the reading of a ROM type optical disk, whereas a tracking method, such as the phase difference method, in which the left and right phase differences are compared during the passage of a recorded pit, enables adequate tracking error signals to be obtained even when the tracking pitch is narrow, since recorded pits will not exist when the original of the ROM type optical disk or a RAM type optical disk is to be recorded, the phase difference method, etc., cannot be used. Thus the push-pull method, with which tracking is performed so that the intensity variations, due to the returning of the plus and minus diffracted primary light from previously formed tracks, will be equal, is used in such cases.
As shown in FIG. 10, with the push-pull method, the modulation factor is determined by the area of the portion (hatched portion) of the diffraction spot image on the light receiving surface of the photodetector of the pickup at which the plus/minus primary light, that is spread in the radial direction by the track, overlaps with the zero-order of light that returns directly. Whereas a relatively large modulation factor can be obtained with a circular aperture as shown in FIG. 10A, when the aperture is elliptical as shown in FIG. 10B, the area of the above-mentioned portion of overlap decreases and the modulation factor of the tracking error signal that is obtained thus decreases. This decrease in modulation factor becomes especially significant in cases where the track pitch is narrowed and the diffraction spot is moved outward in the radial direction.
Thus with an optical system in which the numerical aperture in the radial direction is made greater than the numerical aperture in the time axis direction, when the form of aperture is an elliptical aperture, stable tracking cannot be performed readily in comparison to the case where the aperture is a circular aperture.
The invention has been made in view of the above-described point and an object thereof is to provide an optical pickup device with which good tracking performance can be maintained by preventing a decrease in the area of overlap in the diffraction spot on the photodetector and thereby preventing the decrease in the level of push-pull tracking errors.
The optical pickup device of the present invention is an optical pickup device, having one or a plurality of aperture restriction means that are disposed in an optical path of a light beam that is emitted from a light source and illuminated onto an optical disk, wherein the aperture restriction means restricts a cross-sectional shape of the light beam corresponding to an aperture in a time axis direction in comparison to that corresponding to an aperture in a radial direction and has a pair of first partial arcs restricting the light beam in the radial direction of the optical disk.
The invention also provides an optical pickup device, having one or a plurality of aperture restriction means that are disposed in the optical path of a light beam that is emitted from a light source and illuminated onto the optical disk via an objective lens, wherein the aperture restriction means makes a numerical aperture of the objective lens in the time axis direction of the optical disk smaller than a numerical aperture of the objective lens in the radial direction and has a pair of first partial arcs restricting the light beam in the radial direction of the optical disk.
In the optical pickup device above mentioned, the aperture restriction means has a pair of straight lines for restricting the cross-sectional shape of the light beam in the time axis direction of the optical disk.
In the optical pickup device above mentioned, the aperture restriction means has a pair of lines for restricting the cross-sectional shape of the light beam in the time axis direction of the optical disk so as to be smaller towards end parts thereof.
In the optical pickup device above mentioned, the aperture restriction means has a pair of second partial arcs for restricting the cross-sectional shape of the light beam in the time axis direction of the optical disk, which are concentric with the first partial arcs and are smaller in radius than the first partial arcs.
In the optical pickup device above mentioned, the aperture restriction means has notched parts which reduce the restriction of the cross-sectional shape of the light beam in the time axis direction of the optical disk.
In the optical pickup device above mentioned, the aperture restriction means has a pair of restriction parts formed to be symmetrical with respect to a line parallel to the radial direction.
In the optical pickup device above mentioned, the optical disk has a structure with tracks formed with a predetermined interval provided in between and the partial aperture areas defined by the notched parts are located at positions through which portions of the diffracted light due to the tracks will be transmitted.